This invention relates to carburetors for small internal combustion engines, and in particular to a method for making carburetors for small internal combustion engines such as are used in lawnmowers, snowblowers, chainsaws and the like.
Prior art carburetors have generally been manufactured by die-casting a body and securing a fuel bowl assembly thereto by means of threaded fasteners. The die-cast body is generally made of either aluminum or zinc. Since die-cast aluminum tends often to be porous, die-cast aluminum carburetor bodies must first be impregnated with a special sealing material. After casting, the body must be machined to provide numerous orifices, apertures and the like. While zinc may be used as a die-casting material and is not as porous as aluminum so that it does not need to be impregnated to seal the body, zinc is higher in weight and cost than aluminum and therefore is not a preferred material. In small internal combustion engines, especially those which are used in hand held or easily maneuvered appliances, such as leafblowers, lightweight snowblowers and the like, it is particularly important that the weight of the carburetor be kept to a minimum.
The fuel bowl of prior art carburetor assemblies was generally either made of cast metal or was molded from plastic. Prior art carburetors have also been provided with molded plastic bodies in an attempt to reduce machining and the overall number of separate components required for a carburetor. By manufacturing the carburetor from plastic, many of the details which would normally be machined may be molded in. However, it is difficult to mold certain of the orifices and other features which must be held to required close tolerances such as, for instance, 0.002 of an inch on a 0.250 dimension or larger. Two items which are particularly difficult to mold in plastic carburetor bodies are the throttle bore and the throttle shaft bore. Both of these bores must be held to very tight tolerances and their alignment to each other in the carburetor body is critical. Good performance of a carburetor requires a true throttle bore, especially in a full progression carburetor. Even if close tolerances can be held during the plastic molding process, in time, after the plastic structure has been subjected to thermal cycling and/or stress under load, the plastic material tends to deform due to plastic creep and the tolerance limits are therefore exceeded. Manufacturers have attempted to avoid this problem by eliminating certain functions from the carburetor such as, for instance, an idle system, thereby both limiting performance capabilities and avoiding the need for precision bores.
In some plastic carburetor structures, the throttle bore, throttle shaft bore and idle progression holes are machined in an aluminum portion of the carburetor in order to insure close tolerances. In another attempt to improve performance of molded plastic carburetors, high quality glass, reinforced plastic, or mineral filled plastic materials have been used. However, such filler materials make drilling and machining of the plastic carburetor very difficult. Furthermore, the cost of high quality plastic can be as much as the cost of aluminum.
Still another problem with prior art plastic carburetors has been that the performance of some plastics will deteriorate by contact with gasoline, gasoline/alcohol blends, and especially decomposing gasoline which generates acids and peroxides. Thus it is desired to provide a carburetor for small internal combustion engines which is not only low in cost and is simple to manufacture but has excellent performance, is simple to assemble, and easy to service.